Ab initio kinetic modeling of gas phase radical reactions

AB INITIO KINETIC MODELING OF GAS PHASE RADICAL REACTIONS SUN WENJIE NATIONAL UNIVERSITY OF SINGAPORE 2010 AB INITIO KINETIC MODELING OF GAS PHASE RADICAL REACTIONS SUN WENJIE (B. Eng., Dalian University of Technology, China) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL & BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2010 ACKNOWLEDGEMENTS Firstly, I would like to sincerely express my gratitude to my supervisor, Dr. Mark Saeys, for his support, guidance, comments and suggestions throughout my whole Ph.D studies, which helped me to become a better researcher. I would sincerely like to thank my group members such as Xu Jing, Tan Kong Fei, Hong Won Keon, Chua Yong Ping Gavin, Fan Xuexiang, Zhuo Mingkun, Su Mingjuan, Ravi Kumar Tiwari, for their help, support and encouragement throughout my research work. I am especially thankful for our collaborators Prof. Liya Yu and Dr. Liming Yang. I am grateful to all the technical staff and lab officers for their supports. I would like to thank the Department of Chemical & Biomolecular Engineering, National University of Singapore for providing me the research scholarship. Finally, special thanks to my families for being there to support me as I pursue my doctorate degree. I am extremely grateful for their love, patience and especially their understanding, which have enabled my doctorate journey to be meaningful and successful. I TABLE OF CONTENTS Acknowledgements I Table of Contents II Summary . VI Symbols and Abbreviations XI List of Tables . XIV List of Figures . XVII Chapter Introduction . Chapter Ab initio study of gas phase radical reactions 2.1 Introduction . 2.2 Fundamentals in quantum chemistry 2.3 Ab initio calculations of the enthalpy of formation, entropy and heat capacity 11 2.3.1 Ab initio calculations of the enthalpy of formation . 16 2.3.2 Ab initio calculations of entropy and heat capacity . 22 2.4 Transition state theory and quantum mechanical tunneling . 24 2.4.1 Conventional transition state theory 25 2.4.2 Variational transition state theory 26 2.4.3 Quantum mechanical tunneling . 30 2.5 Pressure dependence of unimolecular dissociation/recombination reactions . 34 II 2.6 Ab initio calculations of kinetic parameters 38 2.6.1 Ab initio rate coefficients for radical-radical recombination reactions 38 2.6.2 Ab initio rate coefficients for radical additions to olefins 41 2.6.3 Ab initio rate coefficients for hydrogen abstraction reactions . 44 2.7 Summary . 46 2.8 References . 47 Chapter Computational methods . 52 3.1 Introduction . 52 3.2 Computational procedures 52 3.2.1 Geometry optimization and electronic energy calculation 52 3.2.2 Calculation of the internal rotation partition function . 53 3.2.3 Calculation of the enthalpy of formation, entropy and heat capacity 54 3.2.4 Calculation of rate coefficients 55 3.2.5 Calculation of tunneling corrections 56 3.3 References . 57 Chapter Ab initio study of the reaction of carboxylic acid with hydroxyl radicals . 59 4.1 Introduction . 59 4.2 Computational procedures 67 4.3 Ab initio study of the reactions of formic and acetic acids with hydroxyl radicals 76 4.3.1 Kinetics of the reaction of formic acid with hydroxyl radicals . 76 III 4.3.1.1 Geometry and energy calculations 76 4.3.1.2 Tunneling corrections . 86 4.3.1.3 Rate coefficient and selectivity . 92 4.3.2 Kinetics of the reaction of acetic acid with hydroxyl radicals . 94 4.3.2.1 Geometry and energy calculations 94 4.3.2.2 Tunneling corrections . 99 4.3.2.3 Rate coefficient and mechanism . 103 4.4 Ab initio reaction path analysis for the initial hydrogen abstraction from valeric acids by hydroxyl radicals 105 4.4.1 Geometry and energy calculations . 106 4.4.2 Kinetic parameters and reaction path analysis . 122 4.5 Summary . 131 4.6 References . 134 Chapter Ab initio simulation of an ethane steam cracker 140 5.1 Introduction . 140 5.2 Computational procedures 144 5.2.1 Reaction network . 144 5.2.2 Ab initio calculation of thermodynamic and kinetic parameters . 145 5.2.3 Experimental conditions 150 5.2.4 Sensitivity analysis 151 5.3 Ab initio simulation of an ethane cracker . 152 IV 5.3.1 Thermodynamic and kinetic parameters 152 5.3.2 Simulations of an ethane cracker using high-pressure-limit rate coefficients. . 164 5.3.3 Simulations of an ethane cracker using pressure-dependent rate coefficients. . 171 5.3.4 Sensitivity analysis 174 5.4 Summary . 180 5.5 References . 182 Chapter Conclusions and outlook 186 Publications . 193 V SUMMARY Kinetic modeling of gas phase radical reactions plays an important role in understanding various atmospheric and biological processes such as the fate of volatile organic compounds and in the design and optimization of important industrial chemical processes such as combustion, radical polymerization, and pyrolysis. Experimental kinetic studies of low temperature radical chemistry in the atmosphere and of high temperature radical reactions in industrial chemical processes remain challenging due to the complexity of the reacting systems and because of the short lifetime of the radical intermediates. To test the predicting capabilities of ab initio calculations for such gas phase radical reactions, we modeled the low temperature atmospheric oxidation of carboxylic acids by hydroxyl radicals and simulated the high temperature industrial steam cracking of ethane. The oxidation of formic and acetic acid by hydroxyl radicals was studied to develop an ab initio computational procedure to accurately predict reaction rate coefficients and selectivities for this family of reactions. For the reaction of formic acid with hydroxyl radicals, activation barriers calculated with the computationally efficient CBS-QB3 method are 14.1 and 12.4 kJ/mol for the acid and for the formyl channel, respectively, and are within 3.0 kJ/mol of values obtained with the computationally VI more demanding W1U method. Multidimensional quantum tunneling significantly enhances the rate coefficient for the acid channel and is responsible for the dominance of the acid channel at 298 K, despite its higher barrier. At 298 K, tunneling correction factors of 339 and 2.0 were calculated for the acid and the formyl channel using the Small Curvature Tunneling method and the CBS-QB3 potential energy surface. The importance of multidimensional tunneling for the acid channel can be attributed to the strong reaction path curvature of the minimum energy path due to coupling between the reaction coordinate and the H-O-H bending modes. Such couplings might also be relevant for biological systems where hydrogen bond networks are prevalent. The standard Wigner, Eckart, and Zero Curvature Tunneling methods only account for tunneling along the reaction path and hence severely underestimate the importance of tunneling for the acid channel. The resulting reaction rate coefficient of 0.98×105 m3/(mol·s) at 298 K is within a factor to of experimental values. For acetic acid, an 11.0 kJ/mol activation barrier and a large tunneling correction factor of 199 were calculated for the acid channel at 298 K. Two mechanisms compete for hydrogen abstraction at the methyl group, with activation barriers of 11.9 and 12.5 kJ/mol and tunneling correction factors of 9.1 and 4.1 at 298 K. The resulting rate coefficient of 1.2×105 m3/(mol·s) at 298 K and branching ratio of 94 % compare again well with experimental data. VII Using the ab initio computational procedure developed for the oxidation of formic and acetic acids, we studied the initial rate and selectivity of the oxidation of valeric acid, C4H9COOH, i.e., the selectivity between abstraction of hydrogen atoms at the acid, α, β, γ and methyl positions. Valeric acid was selected as a representative linear carboxylic acid, and allows quantifying the selectivity between the acid, α-, β-, γ-, and methyl-channel required to begin understand the degradation mechanism of carboxylic acids in the troposphere. At the high-pressure-limit, an overall rate coefficient at 298 K of 4.3×106 m3/(mol∙s) was calculated and the dominant pathways are abstraction at the β, the γ and, to a lesser extent, the acid position, with a selectivity of 55, 28 and %, respectively. This differs from the high selectivity for the acid channel for formic and acetic acid, and from the thermodynamic preference for the α position, but is consistent with the experimentally observed selectivity for abstraction at the β and γ position in larger organic acids. Interestingly, the transition states for abstraction at the β and γ position are characterized by a hydrogen bound, 7or 8-membered ring, e.g., [··H···βC-αC-C=O···HO··]. The rate and selectivity of the oxidation are controlled by the strength of this hydrogen bond between the acid group and the hydroxyl radical in the different transition states, and not correlate with the stability of the products. At 298 K and below 0.1 atm, the collision frequency becomes insufficient to stabilize the pre-reactive complexes, and the reaction becomes chemically activated. However, the reaction rate and the selectivity remain largely unaffected by this mechanistic change. VIII From Figure 5.4, it is clear that the normalized yield change coefficients for C2H6, C2H4, and H2 are smaller than the coefficients for CH4, C2H2, and C3H6. The predicted C2H6, C2H4, and H2 yields are hence less sensitive to the values of the rate coefficients and are mainly determined by the thermodynamic parameters in the kinetic model. Indeed, increasing the rate coefficients for any of the reaction pairs in the model by a factor two changes the predicted yields for those three products by less than 10 %. The C2H6 yield shows a modest sensitivity to the rate coefficients of the C2H6 = ·CH3 + ·CH3 initiation reaction and to the rate coefficients of the ·C2H5 + ·C2H5 = C4H10 reaction. Increasing both the forward and reverse rate coefficient of the C2H6 = ·CH3 + ·CH3 reaction increases the concentration of ·CH3 radicals along the reactor. This increases the C2H6 conversion, and also the yields of C2H4, CH4, C3H6, and C2H2. The yield of C2H4 is most sensitive to the rates of the hydrogen abstraction reactions, C2H6 + ·H = ·C2H5 + H2 and also to the rates of the ·C2H5 + ·C2H5 = C4H10 reactions, which are a net consumer of ·C2H5 radicals. The CH4, C2H2 and C3H6 yields are much more sensitive to the calculated transition state properties, with normalized yield change coefficients of 0.52, −0.28, and −0.40, respectively. A 10 % increase in the rates of the C2H6 = ·CH3 + ·CH3 reactions is calculated to change the CH4 yield by 5.2 %, while a 10 % increase in the rates of the C2H6 + ·H = ·C2H5 + H2 reactions decreases the CH4 yield by 5.1 %. Indeed, the yield 178 of CH4 will decrease while the H2 yield will increase if more C2H6 is consumed by hydrogen abstraction by ·H radicals. The kinetics of the C2H4 + ·H = ·C2H5 reactions are also important for the CH4 yield, as also reported before (Sundaram and Froment, 1978). C3H6 is mainly formed via the β-scission of ·C3H7 radicals and the C3H6 yield is sensitive to the kinetics of these reactions. Again, this reaction was identified by Matheu and Grenda (2005a) and by Van Geem et al. (2004). The normalized yield change coefficient of −0.40 indicates that a 2.0 kJ/mol increase in the enthalpy of formation of the transition state for this reaction will increase the C3H6 yield by 11 %. Finally, the sensitivity analysis identifies reactions involving ·C2H3 radicals, i.e., C2H4 + ·H = H2 + ·C2H3 and ·C2H3 = C2H2 + ·H, as kinetically important to predict accurate C2H2 yields. Again, this finding is consistent with literature reports (Matheu and Grenda, 2005a). 179 5.4 Summary An ab initio kinetic model consisting of 150 reversible elementary reactions and involving 20 species was constructed to simulate steam cracking of ethane. The thermodynamic and kinetic parameters in the model were obtained from CBS-QB3 and W1U quantum chemical calculations in combination with transition state theory. The importance of pressure-dependent rate coefficients was evaluated using the QRRK-MSC approach, but was found to be relatively minor under typical industrial ethane steam cracking conditions. The accuracy of the ab initio kinetic model was tested for three operating conditions reported in the literature; an industrial reactor with a conversion of 60 % and a residence time of 0.93 s, a pilot-scale reactor with a similar conversion, and an industrial split coil reactor with a conversion of 51 % and a residence time of 1.2 s. C2H6 conversions, C2H4 and H2 yields were predicted with an accuracy of better than % for the cases tested. The predicted yields of CH4, C2H2 and C3H6 were found to be particularly sensitive to the accuracy of the kinetic parameters. Despite this high sensitivity, the predicted CH4 yields of 3.3, 2.3 and 2.8 wt% are comparable with experimental yields of 3.4, 3.0 and 2.2 wt%, respectively. Predicted C2H2 yields are somewhat high, which could be attributed to the limited size of the reaction network. Finally, to put the accuracy of the predicted yields and conversions into perspective, it should be noted that the MAD of 1.9 kJ/mol between 180 the CBS-QB3 and the experimental enthalpies of formation translates to a 26 % uncertainty in the predicted equilibrium coefficients at 1000 K. This work hence illustrates that standard state-of-the-art computational chemistry calculations have become sufficiently accurate to begin to predict conversions and selectivities for large scale industrial processes such as ethane steam cracking. An extension to other complex radical reactions such as combustion, radical polymerization, and atmospheric chemistry can be envisioned. Ethane steam cracking was selected to limit the size of the reaction network, but the construction of a predictive ab initio kinetic model for naphtha and vacuum gasoil feedstock is in principle possible. For such large reaction networks, automated network generation (Clymans and Froment, 1984; Broadbelt and Pfaendtner, 2005; Van Geem et al., 2006) becomes indispensable, and group contribution methods (Willems and Froment, 1988; Saeys et al., 2004; Saeys et al., 2006), group additivity methods (Benson, 1976; Cohen, 1992; Sumathi et al., 2001; Sabbe et al., 2005; Sabbe et al., 2008) and structure-activity relationships (Broadbelt et al., 1994; Broadbelt and Pfaendtner, 2005) will be required to keep the number ab initio calculations tractable. 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The main findings of this study are summarized as follows. First, the oxidation of formic and acetic acids by hydroxyl radicals was studied as a model for the oxidation of larger carboxylic acids using ab initio calculations. For formic acid, the CBS-QB3 effective activation barriers of 14.1 and 12.4 kJ/mol were calculated for the acid and for the formyl channel, respectively, are within kJ/mol of benchmark W1U and large basis set CCSD(T) values. Tunneling was found to significantly enhance the rate coefficient for the acid channel and is responsible for the dominance of the acid channel at 298 K. At 298 K, SCT correction factors of 339 and 2.0 were calculated for the acid and the formyl channels using the CBS-QB3 potential energy surface. The Wigner, Eckart, and zero-curvature tunneling methods that not account for the reaction path curvature coupling significantly underestimate the importance of tunneling for the acid channel. The calculated overall 186 rate coefficient at 298 K, 0.98×105 m3/(mol∙s), is within a factor to of experimental values. For acetic acid, a lower barrier of 11.0 kJ/mol and a lower SCT tunneling correction factor of 199 were calculated for the acid channel. Two reaction paths compete for hydrogen abstraction at the methyl group in acetic acid, with activation barriers of 11.9 kJ/mol and 12.5 kJ/mol and tunneling correction factors of 9.1 and 4.1 at 298 K. The resulting overall rate coefficient at 298 K, 1.2×105 m3/(mol∙s), and branching ratio of 94 % compare well with experimental data. Based on the benchmark study of the oxidation of formic and acetic acids by hydroxyl radicals, ab initio calculations were also performed for the initial hydrogen abstraction from organic acids by hydroxyl radicals. To quantify the rate and selectivity of these reactions, the abstraction of hydrogen atoms at the acid, α, β, γ and methyl (δ) positions was studied for valeric acid, C4H9COOH, using ab initio calculations. At the high-pressure limit, an overall rate coefficient at 298 K of 4.3×106 m3/(mol∙s) was calculated. The dominant pathways are abstraction at the β, the γ, and, to a lesser extent, the acid positions with a selectivity of 55, 28, and %, respectively, at 298 K. This differs from the high selectivity for the acid channel for formic and acetic acids and from the thermodynamic preference for abstraction at the α position, 187 but it is consistent with the experimentally observed preference for the β and the γ positions in larger organic acids. The rate and selectivity are controlled by the strength of hydrogen bonds between the acid group and the attacking hydroxyl radical in the different transition states and not correlate with the stability of the products. This hydrogen bond stabilizes the transition state and leads to ring structures. The reaction between a hydroxyl radical and a carboxylic acid begins by the formation of a hydrogen-bonded pre-reactive complex. The hydrogen bond is still present in the transition state and determines the selectivity of the reaction. Abstraction of α hydrogen atoms is difficult because its proximity to the acid group does not allow the formation of a stable ring structure. The transition states for abstraction of the β- and γ-hydrogen atoms are characterized by favorable and member rings. Abstraction of hydrogen atoms further from the acid group requires the formation of larger rings and is hence associated with a high entropy cost. Indeed, two mechanisms were considered for abstraction of the δ hydrogen atoms. At 298 K, the rate coefficient for the mechanism via a transition state without a hydrogen bond is 36 % faster than the mechanism via a transition state with a hydrogen bond. At 298 K and below 0.1 atm, the collision frequency is insufficient to stabilize the prereactive complexes, and the reaction becomes chemically activated. However, the reaction rate and the selectivity are largely unaffected by this mechanistic change. 188 To test the predicting capabilities of ab initio calculations for high temperature industrial processes, the industrial steam cracking of ethane was simulated using an ab initio kinetic model. The reaction network consists of 20 species and 150 reversible elementary reactions. The thermodynamic and kinetic parameters in the model were obtained ab initio CBS-QB3 and W1U calculations in combination with transition state theory and agree well with available experimental data. The accuracy of the first principle kinetic model was tested for the three operating conditions reported in the literature; an industrial reactor with a conversion of 60 % and a residence time of 0.93 s, a pilot-scale reactor with a similar conversion, and an industrial split coil reactor with a conversion of 51 % and a residence time of 1.2 s. Predicted C2H6 conversions, C2H4 and H2 yields are within % of experimental data for the three cases tested. Though CH4 yields and outlet temperatures are particularly sensitive to the accuracy of the kinetic parameters, they are simulated with an accuracy of the better than 10 %. The predicted methane yields of 3.3, 2.3 and 2.8 wt% are comparable with experimental yields of 3.4, 3.0 and 2.2 wt%, respectively. Larger deviations for the C3H6 and C2H2 yields are attributed to the limited size of the reaction network. Finally, to put the accuracy of the predicted yields and conversions into perspective, it should be noted that the MAD of 1.9 kJ/mol between the CBSQB3 and the experimental enthalpies of formation translates to a 26 % uncertainty in the predicted equilibrium coefficients at 1000 K. The effect of total pressure on the 189 rate coefficients was evaluated using the QRRK-MSC approximation, and was found to be relatively minor for the reaction conditions tested. In summary, we have shown that ab initio calculations begin to be capable of predicting the kinetics of complex radical systems with high accuracy. The crucial role of multi-dimensional tunneling in determining the high selectivity of the acid channel in small carboxylic acids, and the importance of hydrogen-bond networks in determining the selectivity in larger organic acids is an intrinsic feature of these low temperature processes. At the other side of the temperature and complexity spectrum, a kinetic model based entirely on high-level quantum chemical calculations was able to accurately predict yields and conversions for the industrial steam cracking of ethane. Outlook A single water molecule as a catalyst for the oxidation reaction of carboxylic acids by hydroxyl radicals In this study, we focus on the oxidation of carboxylic acids by hydroxyl radicals in the atmosphere without the presence of water. Recently, Vöhringer-Martinez et al. (2007) have experimentally proved that a single water molecule catalyzes the oxidation reaction of acetaldehyde with hydroxyl radicals. They found that even a 190 single water molecule accelerates the reaction by forming a hydrogen-bonded complex with CH3CHO and hydroxyl radicals and lowering the reaction barrier significantly. In the humid atmosphere where the catalytic effect of water cannot be neglected, the rate and the selectivity of the oxidation of carboxylic acids by hydroxyl radicals may also change. Therefore, ab initio study of the oxidation of valeric acid by hydroxyl radicals in water vapor is suggested for future work. The rate and the selectivity of the reaction should be able to be evaluated with water as a catalyst. Ab initio study of the oxidation of dicarboxylic acids by hydroxyl radicals In this study, we have shown that the selectivity of the oxidation reaction of valeric acid by hydroxyl radicals can be attributed to the presence of a hydrogen bond between the acid group and the hydroxyl radical in the transition state. In the atmosphere, dicarboxylic acids are also important constitutes. The presence of two acid groups in dicarboxylic acids may affect the selectivity of the oxidation reaction. Experimental work has been done to study the photooxidation rates and oxidation products among C2-C9 dicarboxylic acids (Yang et al., 2008). It will be very interesting to investigate the oxidation of dicarboxylic acids by hydroxyl radicals using an ab initio approach to help understand the rate and selectivity of the reaction. 191 Modeling of industrial pyrolysis processes using a fully ab initio approach In this study, a steam cracker of light feedstock, ethane, was successfully modeled using a fully ab initio approach with a relatively small size of reaction network. The completeness of the reaction network was found to be extremely important for the simulation of an industrial pyrolysis process. In the future, a complete reaction network should be constructed ab initio to achieve a better prediction of the pyrolysis process. The fully ab initio approach should also be applied to the modeling of pyrolysis of heavier feedstock, such as naphtha, in the future to prove that the simulation of a pyrolysis process is not limited to light feedstock only. References Vöhringer-Martinez, E.; Hansmann, B.; Hernandez-Soto, H.; Francisco, J. S.; Troe, J.; Abel, B. Science 2007, 315, 497−501. Yang, L. M.; Ray, M. B.; Yu, L. E. Atmos. Env. 2008, 42, 868−880. 192 PUBLICATIONS 1. Sun, W. J.; Saeys, M. First principles study of the reaction of formic and acetic acids with hydroxyl radicals. Journal of Physical Chemistry A, 2008, 112, 6918−6928. 2. Sun, W. J.; Yang, L.; Yu, L.; Saeys, M. Ab initio reaction path analysis for the initial hydrogen abstraction from organic acids by hydroxyl radicals. Journal of Physical Chemistry A, 2009, 113, 7852−7860. 3. Sun, W. J.; Saeys, M. Construction of an ab initio kinetic model for industrial ethane pyrolysis. AIChE J. 2010, Published online. 193 [...]... using ab initio calculated thermodynamic and kinetic parameters is still a challenge The objective of this thesis is to test the predicting capabilities of ab initio calculations for gas phase radical reactions Ab initio kinetic studies are applied to 1) low temperature atmospheric oxidation of carboxylic acids by hydroxyl radicals and 2) modeling of the high temperature industrial steam cracking of ethane... summary, ab initio kinetic modeling of gas phase radical reactions was performed in this study using high-level quantum chemical calculations and incorporating corrections to the conventional transition state theory We have shown that ab initio calculations begin to be capable of predicting the kinetics of complex radical systems with high accuracy The successful prediction of the rate and selectivity of. .. Although many ab initio kinetic studies of radical reactions in atmospheric chemistry have been performed, there are gaps between ab initio calculations and experimental measurements Computational procedures for accurate kinetic modeling of many radical reactions in the atmosphere are still lacking Precise kinetic prediction of atmospheric radical chemistry could only be possible if the kinetic and thermodynamic... 11509−11516 7 CHAPTER 2 AB INITIO STUDY OF GAS PHASE RADICAL REACTIONS 2.1 Introduction Kinetic modeling of the low temperature atmospheric oxidation of carboxylic acids by hydroxyl radicals and the high temperature industrial steam cracking of ethane are the main focuses of this thesis The complexity of these reacting systems and the difficulty in experimentally detecting the short-lived radical intermediates... of these processes With the development of quantum chemistry theories and the improvement of computational power, it is now possible to perform kinetic studies using an ab initio approach Successful ab initio kinetic studies of gas phase radical reactions have been reported by many researchers (Alvarez-Idaboy et al., 2001; Ochando-Pardo et al., 2004; Ellingson and Truhlar, 2007) A detailed review of. .. overviewed Finally, ab initio calculations of kinetic parameters for three reaction families, i.e., hydrogen abstraction reactions, radical addition to olefins reactions, and radical- radical recombination reactions, important for both the steam cracking of ethane and the oxidation of carboxylic acids by hydroxyl radicals will be presented 2.2 Fundamentals in quantum chemistry The foundation of quantum chemistry... Experimental kinetic studies of low temperature radical chemistry in the atmosphere and of high temperature radical reactions in industrial chemical processes remain challenging due to the complexity of the reacting system and because of the short lifetime of the radical intermediates With the continuous improvement of theories and algorithms in computational chemistry, ab initio calculations begin to be capable... standard ab initio computational chemistry methods has become sufficient to begin to predict the conversion and selectivity for a complex, high temperature gas phase radical process, the industrial steam cracking of ethane was simulated using a fully ab initio kinetic model The modeling shows that the steam cracking of ethane can be predicted with high accuracy with the state -of- art thermodynamic and kinetic. .. standard ab initio computational chemistry methods has become sufficient to begin to predict the conversion and selectivity for a complex, high temperature gas phase radical process, the industrial steam cracking of ethane was modeled using a fully ab initio kinetic model Our reaction network consists of 20 species smaller than C5 and 150 reversible elementary reactions and includes all possible reactions. .. of ethane The oxidation of formic and acetic acid by hydroxyl radicals was studied to develop an ab initio computational procedure to accurately predict reaction rate coefficients and selectivities for this family of reactions The rate coefficients for the oxidation of formic and acetic acids by hydroxyl radicals can be calculated within a factor of 4 of experimental values Ab initio calculations also . AB INITIO KINETIC MODELING OF GAS PHASE RADICAL REACTIONS SUN WENJIE NATIONAL UNIVERSITY OF SINGAPORE 2010 AB INITIO KINETIC MODELING OF GAS PHASE RADICAL. dependence of unimolecular dissociation/recombination reactions 34 III 2.6 Ab initio calculations of kinetic parameters 38 2.6.1 Ab initio rate coefficients for radical- radical recombination reactions. XIV List of Figures XVII Chapter 1 Introduction 1 Chapter 2 Ab initio study of gas phase radical reactions 8 2.1 Introduction 8 2.2 Fundamentals in quantum chemistry 9 2.3 Ab initio calculations